Over 4 million individuals were newly infected with HIV in 2006 with sexual transmission the predominant mode of infection worldwide, highlighting the need for effective prevention strategies. Unfortunately clinical trials to date, with the first generation of candidate topical microbicides to block sexual transmission, have been disappointing as both nonoxynol-9 (N-9) and more recently cellulose sulfate (CS) either did not block transmission or actually enhanced transmission. These results highlight the continued need for highly efficacious and safe microbicide candidates. This project will address the safety and efficacy of a new class of specific anti-retrovirals as topical microbicide candidates, integrase inhibitors. The integrase inhibitor, GS-9160, is a potent inhibitor of HIV which has been extensively studied in animals and most recently in a Phase I human trial and has had no significant toxicity. The potential of this drug as a candidate microbicide will be evaluated in two phases. In the R21 phase, a candidate gel formulation of GS-9160 will be generated in collaboration with Gilead Sciences and evaluated for in vitro drug loading and stability. The drug and candidate formulation with favorable loading will be evaluated in cervical and vaginal epithelial cell monolayers and cervicovaginal explants for release and uptake, cytotoxicity and efficacy against primary and laboratory isolates. The parallel evaluation of gene expression induced by formulated GS-9160 in human and rhesus macaque (RM) cervicovaginal explants along with a similar analysis of tissue and cervical vaginal lavage (CVL) fluid derived from in vivo RM studies in the R33 phase will validate the cervicovaginal explant model as a screen for host responses in vivo. If the candidate formulation has an acceptable safety profile as determined by the absence of a proinflammatory response (comparable to N-9) and inhibits HIV infection in the explant model, the R33 phase will be initiated with testing of local and systemic pharmacokinetics and toxicity associated with vaginal delivery of formulated GS-9160 in (RM) followed by an efficacy study in RM vaginally challenged with R5 SHIV. The proposed studies will directly address whether integrase inhibitors as a class should be added to the pipeline for microbicide development. In addition, studies proposed will validate the genital explant model as a screen for host responses in vivo Over 4 million individuals were newly infected with HIV in 2006 with sexual transmission the predominant mode of infection worldwide, highlighting the need for effective prevention strategies. Topical microbicides that could be applied by the user to protect against sexual transmission of HIV have to date been disappointing in clinical trials. This proposal exams the topical microbicide potential of a very potent antiretroviral drug that inhibits integration of the virus into host cells. If successful in these studies it would be added to a new generation of topical microbicides in the pipeline that specifically target HIV.